Steel sheet

ABSTRACT

This steel sheet has a predetermined chemical composition, Ex. C that is obtained by Ex. C = (%C) - 12 ( (%Ti*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184} is 0.020% or less, a microstructure at a ¼ depth position of a sheet thickness from a surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder of bainite in terms of area fractions, in the microstructure, the average crystal grain diameter is 10.0 µm or less, the average aspect ratio of crystal grains is 0.30 or more, the standard deviation of a Mn concentration is 0.60 mass% or less, a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state, and a tensile strength is 980 MPa or more.

TECHNICAL FIELD

The present invention relates to a steel sheet.

Priority is claimed on Japanese Patent Application No. 2020-049120, filed in Japan on Mar. 19, 2020, the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, efforts have been being made to reduce carbon dioxide emission in a number of fields from the viewpoint of the global environment protection. Automobile manufacturers are also actively developing techniques for weight reduction in vehicle bodies for the purpose of fuel consumption reduction. A decrease in the weight of steel to be used, such as a decrease in the sheet thickness of a steel sheet, makes it possible to easily decrease the weight of vehicle bodies. However, in the case of automobiles, improvement in the impact resistance is also emphasized in order to ensure passenger safety, and thus weight reduction in vehicle bodies by a decrease in the weight of steel to be used or the like, which is easy, cannot be adopted, and weight reduction in vehicle bodies is not easy. Accordingly, studies are underway to thin members using high strength steel sheets in order to satisfy both weight reduction in vehicle bodies and collision resistance. Incidentally, steel sheets to be applied to vehicle components are formed into component shapes, and, normally, the formability deteriorates as the strengths of the steel sheets increase. Therefore, there is a strong desire for steel sheets to be applied to vehicle components to have both a high strength and excellent formability. Specifically, for steel sheets that are used for inner sheet members, structural members, suspension members, and the like of automobiles, stretch flanging (hole expansion) or bending is often used, and thus the steel sheets need to have a high strength and to be excellent in terms of elongation, stretch flangeability and bending workability.

For example, as described in Patent Document 1, as a steel sheet from which excellent elongation can be obtained, a dual-phase steel sheet (hereinafter, DP steel) composed of a composite structure of soft ferrite and hard martensite is known. However, the DP steel sheet is excellent in terms of elongation, but cracks occur in some cases due to the formation of voids in the interface between ferrite and martensite, which have significantly different hardness, and thus there is a case where the DP steel sheet is poor in terms of stretch flangeability or bending workability.

In addition, Patent Document 2 proposes a high strength hot-rolled steel sheet that is obtained by setting the cooling rate in a temperature range from the solidification of a slab to 1300° C. to 10 to 300° C./min and, after finish rolling, coiling the slab at 500° C. or higher and 700° C. or lower and has a steel structure composed of a ferrite single phase and a tensile strength of 1180 MPa or more. Patent Document 2 discloses that the high strength hot-rolled steel sheet is excellent in terms of the bending workability. However, the high strength hot-rolled steel sheet described in Patent Document 2 is manufactured by reheating a slab without cooling the slab to lower than 900° C. where ferrite begins to be formed and hot-rolling the slab. Therefore, there is a problem in that segregation formed during solidification is not sufficiently reduced and there is a case where the bending workability is not stable. In addition, in Patent Document 2, the stretch flangeability is not taken into account.

Patent Document 3 proposes a method for manufacturing a steel sheet having a ferrite area fraction of 80% or more and a tensile strength of 980 MPa or more by completing hot rolling within five hours after continuous casting to form a solid solution of Ti exceeding the solubility in y and precipitating fine TiC together with ferritic transformation during coiling at 550° C. or higher and 700° C. or lower and a high strength hot-rolled steel sheet that is obtained by the manufacturing method. However, even in Patent Document 3, since continuous casting through the completion of hot finish rolling is performed in an austenite region to suppress the precipitation of coarse TiC, there has been a case where the bending workability deteriorates due to Mn segregation. In addition, in Patent Document 3 as well, similar to Patent Document 2, the stretch flangeability is not taken into account.

CITATION LIST Patent Documents

-   [Patent Document 1] Japanese Unexamined Patent Application, First     Publication No. H6-128688 -   [Patent Document 2] Japanese Unexamined Patent Application, First     Publication No. 2014-194053 -   [Patent Document 3] Japanese Unexamined Patent Application, First     Publication No. 2014-208876

SUMMARY OF THE INVENTION Problems to Be Solved by the Invention

The present invention has been made in consideration of the above-described problems, and an object of the present invention is to provide a steel sheet having a high strength and being excellent in terms of elongation, stretch flangeability and bending workability. Here, the steel sheet of the present invention also includes steel sheets having a cover such as a plating layer on the surface.

Means for Solving the Problem

The present inventors studied steel sheets that are favorable in all of the strength, the elongation, the stretch flangeability and the bending workability. As a result, it was found that a steel sheet having a high strength and being excellent in terms of elongation, stretch flangeability and bending workability can be manufactured by optimizing the chemical composition and manufacturing conditions to control the microstructure of the steel sheet and Mn segregation and controlling the precipitation form of a Ti-based carbide.

The present invention has been made based on the above-described finding, and the gist of the present invention is as described below.

[1] A steel sheet according to an aspect of the present invention contains, as a chemical composition, by mass%, C: 0.050% to 0.250%, Si: 0.005% to 2.000%, Mn: 0.10% to 3.00%, P: 0.100% or less, S: 0.0100% or less, sol. Al: 0.001% to 1.00%, Ti: 0.150% to 0.400%, N: 0.0010% to 0.0100%, Nb: 0% to 0.100%, V: 0% to 1.000%, Mo: 0% to 1.000%, Cu: 0% to 1.00%, Ni: 0% to 1.00%, Cr: 0% to 2.00%, W: 0% to 1.000%, B: 0% to 0.0020%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100% and Bi: 0% to 0.0200% with a remainder of Fe and impurities, in which Ex. C obtained by the following formula (1) is 0.020% or less, a microstructure at a ¼ depth position of a sheet thickness from a surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder of bainite in terms of area fractions, in the microstructure, the average crystal grain diameter is 10.0 µm or less, the average aspect ratio of crystal grains is 0.30 or more, the standard deviation of a Mn concentration is 0.60 mass% or less, a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state, and a tensile strength is 980 MPa or more.

$\begin{array}{l} {\text{Ex}\text{. C =}\left( \text{\%C} \right)\text{- 12}\left\{ {{\left( \text{\%Ti*} \right)/48}\,\text{+}\,{\left( \text{\%V} \right)/51}\,\text{+}\,{\left( \text{\%Nb} \right)/93}} \right)} \\ {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{+}\,{\left( \text{\%Mo} \right)/96}\text{+}\,\left( {\left( \text{\%W} \right)/184} \right\}} \end{array}$

Here, “%Ti*” in the formula (1) is obtained from the following formula (2).

%Ti* = %Ti - 48  ×  {(%N)/14)+((%S)/32)

%C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%.

The steel sheet according to [1] may contain, as the chemical composition, by mass%, one or more selected from the group consisting of Nb: 0.001% to 0.100%, V: 0.005% to 1.000%, Mo: 0.001% to 1.000%, Cu: 0.02% to 1.00%, Ni: 0.02% to 1.00%, Cr: 0.02% to 2.00%, W: 0.02% to 1.000%, B: 0.0001% to 0.0020%, Ca: 0.0002% to 0.0100%, Mg: 0.0002% to 0.0100%, REM: 0.0002% to 0.0100%, and Bi: 0.0001 % to 0.0200%.

The steel sheet according to [1] or [2], in which a plating layer may be formed on a surface.

The steel sheet according to [3], in which the plating layer may be a hot-dip galvanized layer.

The steel sheet according to [4], in which the hot-dip galvanized layer may be a hot-dip galvannealed layer.

Effects of the Invention

According to the above-described aspect of the present invention, it is possible to provide a steel sheet having a high strength and being excellent in terms of elongation, stretch flangeability and bending workability. The steel sheet of the present invention is preferable as a material that is used in uses for automobiles, home appliances, mechanical structures, construction and the like, and, in particular, when the steel sheet is used as a material for components such as inner sheet members, structural members, suspension members, and the like of automobiles, not only is a contribution made to weight reduction in vehicle bodies and improvement in impact resistance but the steel sheet is also easily worked into component shapes.

Embodiments of the Invention

Hereinafter, a steel sheet according to an embodiment of the present invention (the steel sheet according to the present embodiment) will be described below in detail. However, the present invention is not limited only to the configuration disclosed in the present embodiment and can be modified in a variety of manners within the scope of the gist of the present invention.

First, the chemical composition of the steel sheet according to the present embodiment will be described.

Numerical value limiting ranges expressed below using “to” include the values at both ends as the lower limit and the upper limit in the ranges. However, numerical values expressed with ‘less than’ or ‘more than’ are not included in numerical value ranges. In the following description, “%” regarding the chemical composition of the steel sheet indicates “mass%” in all cases.

Chemical Composition of Steel Sheet C: 0.050% To 0.250%

C is an element that bonds to Ti or the like to form a carbide, thereby increasing the tensile strength of steel. When the C content is less than 0.050%, it becomes difficult to obtain a tensile strength of 980 MPa or more. Therefore, the C content is set to 0.050% or more. The C content is preferably set to 0.070% or more.

On the other hand, when the C content is more than 0.250%, there is a concern about a deterioration of the weldability. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less, more preferably 0.200% or less and still more preferably 0.180% or less.

Si: 0.005% To 2.000%

Si is an element having an action of increasing the tensile strength of steel by solid solution strengthening and the enhancement of hardenability. In addition, Si is an element that also has an action of suppressing the precipitation of cementite. When the Si content is less than 0.005%, it becomes unlikely for the above-described action to be exhibited. Therefore, the Si content is set to 0.005% or more. The Si content is preferably 0.010% or more.

On the other hand, when the Si content is more than 2.000%, the surface properties of the steel sheet significantly deteriorate due to surface oxidation in a hot rolling step. Therefore, the Si content is set to 2.000% or less. The Si content is preferably 1.500% or less and more preferably 1.300% or less.

Mn: 0.10% To 3.00%

Mn is an element having an action of increasing the tensile strength of steel by solid solution strengthening and the enhancement of hardenability. When the Mn content is less than 0.10%, ferritic transformation is excessively promoted, and a Ti-based carbide is coarsely precipitated together with the ferritic transformation at high temperatures. In this case, it becomes difficult to obtain a tensile strength of the steel sheet of 980 MPa or more. Therefore, the Mn content is set to 0.10% or more. The Mn content is preferably 0.30% or more and more preferably 0.50% or more.

On the other hand, when the Mn content is more than 3.00%, ferritic transformation and bainitic transformation are delayed, and a desired ferrite area fraction cannot be obtained. In this case, the elongation deteriorates, and the formation of MA degrades the stretch flangeability or the bending workability. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, more preferably 2.00% or less and still more preferably 1.50% or less.

Sol. A1: 0.001% To 1.00%

Al is an element having an action of cleaning steel by deoxidation in a steelmaking stage. When the sol. Al content is less than 0.001%, it becomes difficult to exhibit the above-described action. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.01% or more, more preferably 0.02% or more and still more preferably 0.03% or more.

On the other hand, even when the sol. Al content is set to more than 1.00%, the effect of the above-described action is saturated, and the refining cost increases. Therefore, the sol. Al content is set to 1.00% or less. The sol. Al content is preferably 0.80% or less and more preferably 0.60% or less. sol. Al refers to acid-soluble Al.

Ti: 0.150% To 0.400%

Ti is an element that bonds to C to form a Ti-based carbide and contributes to increase in the tensile strength of the steel sheet. In addition, Ti is an element having an action of refining the microstructure by forming a Ti nitride to suppress the coarsening of austenite during the reheating and hot rolling of a slab. When the Ti content is less than 0.150%, it becomes difficult to obtain a tensile strength of 980 MPa or more due to the lack of the precipitation hardening amount. Therefore, the Ti content is set to 0.150% or more. The Ti content is preferably 0.170% or more, more preferably 0.190% or more and still more preferably 0.210% or more.

On the other hand, when the Ti content becomes excessive, a coarse Ti-based carbide remains in austenite in an undissolved state, which degrades the elongation or the bending workability, and the amount of a Ti-based carbide having a Baker-Nutting orientation relationship contributing to the strength, which decreases the strength. Therefore, the Ti content is set to 0.400% or less. The Ti content is preferably 0.380% or less and more preferably 0.350% or less.

N: 0.0010% To 0.0100%

N is an element having an action of refining the microstructure by forming a Ti nitride to suppress the coarsening of austenite during the reheating and hot rolling of a slab. When the N content is less than 0.0010%, it becomes difficult to exhibit the above-described action. Therefore, the N content is set to 0.0010% or more. The N content is preferably 0.0015% or more and more preferably 0.0020% or more.

On the other hand, when the N content is more than 0.0100%, a coarse Ti nitride is formed, and the stretch flangeability of the steel sheet deteriorates. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0060% or less and more preferably 0.0050% or less.

P: 0.100% or Less

P is an element that is contained in steel as an impurity and has an action of degrading the stretch flangeability or bending workability of the steel sheet. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.060% or less, more preferably 0.040% or less and still more preferably 0.020% or less. P is mixed from a raw material as an impurity, and the lower limit thereof is not particularly limited, but the P content is preferably as small as possible from the viewpoint of ensuring the bending workability. However, when the P content is excessively decreased, the manufacturing cost increases. From the viewpoint of the manufacturing cost, the P content is preferably 0.001% or more and more preferably 0.005% or more.

S: 0.0100% or Less

S is an element that is contained in steel as an impurity and has an action of degrading the stretch flangeability or bending workability of the steel sheet. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, more preferably 0.0060% or less and still more preferably 0.0030% or less. S is mixed from the raw material as an impurity, and the lower limit thereof is not particularly limited, but the S content is preferably as small as possible from the viewpoint of ensuring the bending workability. However, when the S content is excessively decreased, the manufacturing cost increases. From the viewpoint of the manufacturing cost, the S content is preferably 0.0001% or more, more preferably 0.0005% or more and still more preferably 0.0010% or more.

The remainder of the chemical composition of the steel sheet according to the present embodiment includes Fe and impurities. In the present embodiment, the impurity means a substance that is mixed from ore as a raw material, a scrap, the manufacturing environment or the like and is allowed to an extent that the steel sheet according to the present embodiment is not adversely affected.

The steel sheet according to the present embodiment may contain the following optional elements instead of some of Fe. Since the steel sheet according to the present embodiment is capable of solving the problems even when the optional elements are not contained, the lower limit of the amount of the optional elements is 0%.

Nb: 0% To 0.100%

Nb is an optional element. Nb is an element having effects on the suppression of the coarsening of the crystal grain diameters of the steel sheet and an increase in the tensile strength of the steel sheet by the refinement of the ferrite grain diameters or precipitation hardening attributed to the precipitation of Nb as NbC. In order to obtain these effects, the Nb content is preferably set to 0.001 % or more. The Nb content is more preferably 0.005% or more and still more preferably 0.010% or more.

On the other hand, when the Nb content exceeds 0.100%, the above-described effects are saturated, and there is a concern about an increase in the rolling force during finish rolling. Therefore, in a case where Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.060% or less and more preferably 0.030% or less.

V: 0% To 1.000%

V is an optional element. V is an element having effects on an increase in the tensile strength of the steel sheet by the formation of a solid solution in steel and increase in the tensile strength of the steel sheet by precipitation hardening attributed to the precipitation of V as a carbide, a nitride, a carbonitride or the like in steel. In order to obtain these effects, the V content is preferably set to 0.005% or more. The V content is more preferably 0.010% or more and still more preferably 0.050% or more.

On the other hand, when the V content exceeds 1.000%, a carbide is likely to become coarse and there is a case where the bending workability deteriorates. Therefore, in a case where V is contained, the V content is set to 1.000% or less. The V content is preferably 0.800% or less and more preferably 0.600% or less.

Mo: 0% to 1.000%

Mo is an optional element. Mo is an element having effects on the high-strengthening of the steel sheet by the enhancement of the hardenability of steel and the formation of a carbide or a carbonitride. In order to obtain these effects, the Mo content is preferably set to 0.001% or more. The Mo content is more preferably 0.005% or more, still more preferably 0.010% or more and far still more preferably 0.050% or more.

On the other hand, when the Mo content exceeds 1.000%, there is a case where the cracking sensitivity of a steel material such as a slab is enhanced. Therefore, in a case where Mo is contained, the Mo content is set to 1.000% or less. The Mo content is more preferably 0.800% or less and still more preferably 0.600% or less.

Cu: 0% To 1.00%

Cu is an optional element. Cu is an element having an effect on improvement in the toughness of steel and an effect on an increase in the tensile strength. In order to obtain these effects, the Cu content is preferably set to 0.02% or more.

On the other hand, when Cu is excessively contained, there is a case where the weldability of the steel sheet deteriorates. Therefore, in a case where Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less and more preferably 0.30% or less.

Ni: 0% To 1.00%

Ni is an optional element. Ni is an element having an effect on improvement in the toughness of steel and an effect on an increase in the tensile strength. In order to obtain these effects, the Ni content is preferably set to 0.02% or more.

On the other hand, when Ni is excessively contained, the alloying cost increases, and there is a case where the toughness of the steel sheet in a welded heat-affected zone deteriorates. Therefore, in a case where Ni is contained, the Ni content is set to 1.00% or less. The Ni content is preferably 0.50% or less and more preferably 0.30% or less.

Cr: 0% to 2.00%

Cr is an optional element. Cr is an element having an effect on an increase in the tensile strength by the enhancement of the hardenability of steel. In order to obtain this effect, the Cr content is preferably set to 0.02% or more. The Cr content is more preferably 0.05% or more and still more preferably 0.10% or more.

On the other hand, when the Cr content become excessive, the chemical convertibility deteriorates. Therefore, in a case where Cr is contained, the Cr content is set to 2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00% or less and still more preferably 0.50% or less.

W: 0% To 1.000%

W is an optional element. W is an element having an effect on an increase in the tensile strength by the formation of a carbide or a carbonitride. In order to obtain this effect, the W content is preferably set to 0.020% or more.

On the other hand, even when more than a certain amount of W is contained, the effect of the above-described action is saturated, and thus the alloying cost increases. Therefore, in a case where W is contained, the W content is set to 1.000% or less. The W content is preferably 0.800% or less.

B: 0% To 0.0020%

B is an optional element. B is an element having an effect on an increase in the tensile strength of the steel sheet by grain boundary strengthening or solid solution strengthening. In order to obtain this effect, the B content is preferably set to 0.0001 % or more. The B content is more preferably 0.0002% or more.

On the other hand, even when more than 0.0020% of B is contained, not only is the above-described effect saturated, but the alloying cost also increases. Therefore, in a case where B is contained, the B content is set to 0.0020% or less. The B content is more preferably 0.0015% or less.

Ca: 0% To 0.0100%

Ca is an optional element. Ca is an element having an effect on the refinement of the microstructure of the steel sheet by the dispersion of a number of fine oxides in molten steel. In addition, Ca is an element having an effect on improvement in the stretch flangeability of the steel sheet by fixing S in molten steel as spherical CaS to suppress the formation of an elongated inclusion such as MnS. In order to obtain these effects, the Ca content is preferably set to 0.0002% or more. The Ca content is more preferably 0.0005% or more and still more preferably 0.0010% or more.

On the other hand, when the Ca content exceeds 0.0100%, the amount of CaO in steel increases, and there is a case where the toughness of the steel sheet deteriorates. Therefore, in a case where Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0050% or less and more preferably 0.0030% or less.

Mg: 0% To 0.0100%

Mg is an optional element. Similar to Ca, Mg is an element having effects on the suppression of the formation of coarse MnS by the formation of an oxide or a sulfide in molten steel and the refinement of the microstructure of the steel sheet by the dispersion of a number of fine oxides. In order to obtain these effects, the Mg content is preferably set to 0.0002% or more. The Mg content is more preferably 0.0005% or more and still more preferably 0.0010% or more.

On the other hand, when the Mg content exceeds 0.0100%, an oxide in steel increases, and there is a case where the toughness of the steel sheet deteriorates. Therefore, in a case where Mg is contained, the Mg content is set to 0.0100% or less. The Mg content is preferably 0.0050% or less and more preferably 0.0030% or less.

REM: 0% To 0.0100%

REM is an optional element. Similar to Ca, REM is also an element having effects on the suppression of the formation of coarse MnS by the formation of an oxide or a sulfide in molten steel and the refinement of the microstructure of the steel sheet by the dispersion of a number of fine oxides. In the case of obtaining these effects, the REM content is preferably set to 0.0002% or more. The REM content is more preferably 0.0005% or more and still more preferably 0.0010% or more.

On the other hand, when the REM content exceeds 0.0100%, an oxide in steel increases, and there is a case where the toughness of the steel sheet deteriorates. Therefore, in a case where REM is contained, the REM content is set to 0.0100% or less. The REM content is preferably 0.0050% or less and more preferably 0.0030% or less.

Here, REM (rare earth metal) refers to a total of 17 elements including Sc, Y, and lanthanoids. In the present embodiment, the REM content refers to the total amount of these elements.

Bi: 0% to 0.0200%

Bi is an optional element. Bi is an element having an effect on improvement in the formability of the steel sheet by the refinement of the solidification structure. In order to obtain this effect, the Bi content is preferably set to 0.0001% or more. The Bi content is more preferably 0.0005% or more.

On the other hand, when the Bi content exceeds 0.0200%, the above-described effect is saturated, and the alloying cost increases. Therefore, in a case where Bi is contained, the Bi content is set to 0.0200% or less. The Bi content is preferably 0.0100% or less and more preferably 0.0070% or less.

Ex. C: 0.020% or Less

C is precipitated as a Ti-based carbide and contributes to the high-strengthening of the steel sheet. However, when the amount of C contained is larger than the amount of C to be precipitated as a Ti-based carbide, excess C forms pearlite, cementite, MA or the like and consequently degrades the stretch flangeability or the bending workability.

Ex. C that is obtained by the following formula (1) corresponds to the amount of C contained more than the amount of C to be precipitated as a Ti-based carbide. In the steel sheet according to the present embodiment, this Ex. C is set to 0.020% or less. Ex. C is preferably 0.018 or less and more preferably 0.015% or less. The lower limit is not particularly limited.

$\begin{array}{l} {\text{Ex}\text{. C =}\left( \text{\%C} \right)\text{- 12}\left\{ {{\left( \text{\%Ti*} \right)/48}\,\text{+}\,{\left( \text{\%V} \right)/51}\,\text{+}\,{\left( \text{\%Nb} \right)/93}} \right)} \\ {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{+}\,{\left( \text{\%Mo} \right)/96}\text{+}\,\left( {\left( \text{\%W} \right)/184} \right\}} \end{array}$

Here, “%Ti*” in the formula (1) is obtained from the following formula (2).

%Ti* = %Ti - 48  ×  {(%N)/14)+((%S)/32)

%C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%, respectively.

Next, the microstructure of the steel sheet will be described. In the steel sheet according to the present embodiment, the microstructure at a ¼ depth position of the sheet thickness from the surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder including bainite. In addition, in the microstructure, the average crystal grain diameter is 10.0 µm or less, the average aspect ratio of crystal grains is 0.30 or more, and the standard deviation of the Mn concentration is 0.60 mass% or less. In addition, a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state.

Here, the reason for regulating the microstructure at the ¼ depth position of the sheet thickness in the sheet thickness direction from the surface of the steel sheet (a t/4 position from the surface in a case where the sheet thickness is represented by t) is that the microstructure at this position is a typical microstructure of the steel sheet.

-   (Area fraction of ferrite: 60% or more) -   (Area fraction of MA: 0% to 5%) -   (Total area fraction of pearlite and cementite: 0% to 5%)

Remainder: Bainite

Ferrite is required to obtain favorable elongation. When the area fraction is less than 60%, the elongation deteriorates. Therefore, the area fraction of ferrite is set to 60% or more. The area fraction of ferrite is preferably 70% or more, more preferably 80% or more and may be 100% (that is, a ferrite single phase).

There is a case where the microstructure contains, in addition to ferrite, a small amount of MA, which is allowed as long as the area fraction is 5% or less. The area fraction is preferably 4% or less, more preferably 3% or less and most preferably 2% or less. In addition, there is a case where pearlite and cementite are precipitated, which is allowed as long as the total area fraction is 5% or less. The total area fraction is preferably 4% or less, more preferably 3% or less and most preferably 2% or less. When the area fraction of MA is more than 5%, the bending workability and the hole expandability deteriorate. Alternatively, when the area fraction of pearlite and cementite is more than 5%, the hole expandability deteriorates.

In the microstructure, the remainder other than the above-described structures includes bainite. The hardness difference is small between bainite and ferrite that has been precipitation-hardened by a Ti-based carbide. Therefore, bainite has a small effect on the degradation of the hole expandability compared with MA (Martensite-Austenite constituents), pearlite and cementite. Therefore, bainite is contained as the remainder in microstructure.

Average Crystal Grain Diameter: 10.0 µm or Less)

When the average crystal grain diameter is large, the bending workability deteriorates. Therefore, in the microstructure, the average crystal grain diameter is set to 10.0 µm or less. The average crystal grain diameter is preferably 8.0 µm or less. Since the average crystal grain diameter is preferably as small as possible, the lower limit is not particularly limited. However, it is technically difficult to refine crystal grains by ordinary hot rolling such that the average crystal grain diameter becomes less than 1.0 µm. Therefore, the average crystal grain diameter may be set to 1.0 µm or more.

“The average crystal grain diameter” in the present embodiment refers to the average value of crystal grain diameters for which a region that is surrounded by grain boundaries having a crystal orientation difference of 15° or more and has a circle equivalent diameter of 0.3 µm or more in a material having a bcc crystal structure, that is, ferrite, bainite, martensite, and pearlite is defined as a crystal grain, and the crystal grain diameters of residual austenite are not included in the average crystal grain diameter.

Average Aspect Ratio of Crystal Grain: 0.30 or More

In the present embodiment, the average aspect ratio of bcc crystal grains is 0.30 or more. The aspect ratio is a value obtained by dividing the length of the minor axis of a crystal grain by the length of the major axis and has a value of 0 to 1.00. As the average aspect ratio of crystal grains becomes smaller, the crystal grains become flatter, and, as the average aspect ratio becomes closer to 1.00, it is indicated that a crystal grain becomes more equiaxial. When the average aspect ratio of the crystal grains is less than 0.30, there are a number of flat crystal grains, the anisotropy of the material becomes large, and the stretch flangeability and the bending workability deteriorate. Therefore, the average aspect ratio of the crystal grains excluding residual austenite is set to 0.30 or more. As the crystal grains become more equiaxial, the anisotropy becomes smaller, and the workability becomes superior, and thus the average aspect ratio of the crystal grains excluding residual austenite is preferably as close to 1.00 as possible.

In the present embodiment, the average crystal grain diameter, the average aspect ratio of the crystal grains and the area fractions of the microstructure are obtained by the scanning electron microscopic (SEM) observation and the electron back scattering diffraction (EBSD) analysis of the microstructure at the ¼ depth position of the steel thickness from the surface of the steel sheet of a cross section of the steel sheet parallel to a rolling direction and the sheet thickness direction using an EBSD analyzer composed of a thermal field emission scanning electron microscope and an EBSD detector. In a region that is 200 µm long in the rolling direction and 100 µm long in the sheet thickness direction and has the ¼ depth position of the sheet thickness from the surface of the steel sheet at the center, crystal orientation information is acquired at 0.2 µm intervals while differentiating fcc and bcc. Crystal grain boundaries having a crystal orientation difference of 15° or more are specified using the software attached to the EBSD analyzer (“OIM Analysis (registered trademark)” manufactured by AME1EK, Inc.). Regarding the average crystal grain diameter of bcc, the average crystal grain diameter is obtained by defining a region that is surrounded by crystal grain boundaries having a crystal orientation difference of 15° or more and has a circle equivalent diameter of 0.3 µm or more as a crystal grain.

A crystal grain boundary having a crystal orientation difference of 15° or more is mainly a ferrite grain boundary or a block boundary of martensite and bainite. In a method for measuring ferrite grain diameters according to JIS G 0552: 2013, there is a case where a grain diameter is calculated even for a ferrite grain having a crystal orientation difference of less than 15°, and furthermore, a block of martensite or bainite is not calculated. Therefore, as the average crystal grain diameter in the present embodiment, a value obtained by EBSD analysis as described above is adopted. In the EBSD analysis, since the length of the major axis and the length of the minor axis of each crystal grain are also obtained at the same time, the average aspect ratio of bcc crystal grains is also obtained by adopting the present method.

The area fraction of ferrite is measured by the following method. Here, a region that is surrounded by grain boundaries having a crystal orientation difference of 5° or more and has a circle equivalent diameter of 0.3 µm or more is defined as a crystal grain. Among such crystal grains, for crystal grains for which a value that is obtained by an analysis with Grain Average Misorientation analysis equipped in OIM Analysis (GAM value) is 0.6° or less, the area fraction is calculated. The area fraction of ferrite is obtained by such a method. The reason for defining a boundary having a crystal orientation difference of 5° or more as a grain boundary at the time of obtaining the area fraction of ferrite is that there is a case where it is not possible to differentiate different microstructures formed as close variants from the same prior austenite grain.

The area fraction of pearlite and cementite is obtained by observing microstructures revealed by Nital etching with a SEM. The area fraction of MA is obtained by observing a microstructure revealed by LePera etching with an optical microscope. The area fraction may be obtained by an image analysis or may be obtained by a point counting method. For example, for pearlite and cementite, the area fractions may be obtained by the point counting method with lattice spacings of 5 µm after three or more visual fields (100 µm × 100 µm/visual field) in a region at the ¼ depth position of the sheet thickness from the surface of the steel sheet are observed at a magnification of 1000 times. In addition, the area fraction of MA may be obtained by the point counting method with lattice spacings of 5 µm after two or more visual fields (200 µm × 200 µm/visual field) in a region at the ¼ depth position of the sheet thickness from the surface of the steel sheet are observed at a magnification of 500 times.

Standard Deviation of Mn Concentration: 0.60 mass% or Less

The standard deviation of the Mn concentration at the ¼ depth position of the sheet thickness from the surface of the steel sheet according to the present embodiment is 0.60 mass% or less. In such a case, a local unevenness in the tensile strength attributed to Mn segregation is reduced, and it is possible to stably obtain favorable bending workability. The value of the standard deviation of the Mn concentration is desirably as small as possible, but the substantial lower limit is 0.10 mass% due to restrictions in the manufacturing process.

The standard deviation of the Mn concentration can be obtained by collecting a sample such that a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet becomes an observed section, mirror-polishing the observed section and then measuring the ¼ depth position of the sheet thickness from the surface of the steel sheet with an electron probe microanalyzer (EPMA). As the measurement conditions, the acceleration voltage is set to 15 kV, the magnification is set to 5000 times, and a distribution image in a range that is 20 µm long in the rolling direction of the sample and 20 µm long in the sheet thickness direction of the sample is measured. More specifically, the measurement intervals are set to 0.1 µm, and the Mn concentrations at 40000 or more sites are measured. Next, the standard deviation is calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentration.

Ti-Based Carbide

In the steel sheet according to the present embodiment, a carbide containing Ti (Ti-based carbide) is precipitated in ferrite. Ti is an element having a high driving force for the precipitation of a carbide in ferrite, and the control of the content and a heat treatment make it easy to control the precipitation state of a carbide. In addition, the Ti-based carbide also has a high precipitation hardening capability. Here, the Ti-based carbide refers to a carbide having a NaCl-type crystal structure containing Ti. In a case where such a carbide contains Ti, even when a small amount of other carbide-forming alloying elements are contained, the above-described driving force is not significantly weakened, and thus the effect can be obtained. Within the range of the chemical composition that is regulated by the present embodiment, the Ti-based carbide may contain other carbide-forming alloying elements, for example, Mo, Nb, V, Cr and W. Furthermore, even when the Ti-based carbide is a carbonitride in which some of carbon atoms have been substituted with nitrogen atoms, the precipitation state does not change, and thus the effect can be obtained.

Precipitation of Ti-Based Carbides in Ferrite in Semi-Coherent State

In a case where the proportion of Ti-based carbides for which the interface with ferrite is a semi-coherent interface to the Ti-based carbides precipitated in ferrite, which have the Baker-Nutting orientation relationship, is 50% or more, the stretch flangeability of the steel sheet becomes stably favorable. The state where “the Ti-based carbides are precipitated in a semi-coherent state” mentioned in the present embodiment refers to such case. In a case where the Ti-based carbides are not precipitated in a semi-coherent state, the hole expandability deteriorates.

Whether or not the Ti-based carbides having the Baker-Nutting orientation relationship are in a semi-coherent state is determined as described below. That is, an annular dark-field scanning transmission electron microscopic image, for which the detection angle of an annular detector is set to 60 mrad or more and 200 mrad or less in the scanning transmission electron microscopy (magnification: 910,000 times to 5,100 ,000 times), is captured by injecting electron beams into a thin film sample for a transmission electron microscope produced from the ¼ depth position of the sheet thickness from the surface along a [001] orientation of ferrite. When a particle forming a plate-like form having a (100) plane of ferrite in the matrix as a habit plane and a particle forming a plate-like form having a (010) plane of ferrite as a habit plane are regarded as the Ti-based carbides having the Baker-Nutting orientation relationship, a case where the numbers of the crystal planes of a {010} plane of ferrite and a {01-1} plane of the Ti-based carbides that sandwich the habit plane of the (100) plane of the particle forming a plate-like form having a (100) plane of ferrite in the matrix as a habit plane and the habit plane of the (100) plane of the particle forming a plate-like form having a (010) plane of ferrite as a habit plane coincide with each other is determined as a coherent state, and a case where the numbers of the crystal planes do not coincide with each other is determined as the semi-coherent state. In a case where 20 or more Ti-based carbides are observed and 50% or more is in the semi-coherent state, the Ti-based carbides having the Baker-Nutting orientation relationship in steel from which the observed thin film sample for a transmission electron microscope has been collected are determined to be in the semi-coherent state.

Regarding the sizes of the Ti-based carbides, ordinarily, as the carbides become larger, the number density tends to become smaller. In the present invention, from the viewpoint of ensuring the number density of the Ti-based carbides that are precipitated in ferrite to have the Baker-Nutting orientation relationship, the thickness of the Ti-based carbide needs to be 1 nm or more and 5 nm or less.

The thickness of the Ti-based carbide is measured by the following method.

A thin film sample for a transmission electron microscope is produced from the ¼ depth position in the sheet thickness direction from the surface of the steel sheet and observed with a scanning transmission electron microscope (hereinafter, also referred to as “STEM”). In a Ti-based carbide forming sheet surfaces on the (100) plane and the (010) plane of ferrite observed in a STEM image captured by injecting electron beams along the [001] orientation of ferrite, the length of a small side between the sizes of the Ti-based carbide measured along the [100] and [010] orientations of ferrite is regarded as the thickness. In addition, at the time of evaluating the thickness of the Ti-based carbide, a scale is corrected such that the interatomic distance, which is as long as 10 unit lattices, becomes 2.866 nm in each of the [100] orientation and the [010] orientation of ferrite in a site where no precipitates are shown in the image.

Mechanical Properties Tensile Strength: 980 MPa or More

The steel sheet according to the present embodiment has a high strength and is excellent in terms of the elongation, the stretch flangeability, and the bending workability by the control of the microstructure, the precipitation form of the Ti-based carbide and Mn segregation. However, when the tensile strength of the steel sheet is small, an effect on weight reduction in vehicle bodies, rigidness improvement or the like is small. Therefore, the tensile strength (TS) of the steel sheet according to the present embodiment is set to 980 MPa or more. The tensile strength is preferably 1080 MPa or more. Although the upper limit is not particularly regulated; however, as the tensile strength increases, press forming becomes more difficult. Therefore, the tensile strength may be set to 1800 MPa or less.

From the viewpoint of the formability, the steel sheet according to the present embodiment aims at TS × λ, which serves as an index of the balance between the strength and the stretch flangeability, of 50000 MPa·% or more and aims at TS × El, which serves as an index of the balance between the strength and the elongation, of 14000 MPa·% or more. TS × El is more preferably 15000 MPa·% or more. TS x λ is more preferably 55000 MPa·% or more, still more preferably 60000 MPa % or more and far still more preferably 65000 MPa·% or more.

The tensile strength and elongation of the steel sheet are evaluated by the tensile strength and the total elongation at fracture (El) using a No. 5 test piece regulated in JIS Z 2241: 2011. The stretch flangeability of the steel sheet is evaluated with the limiting hole expansion ratio (λ) regulated in JIS Z 2256: 2010.

Manufacturing Method

The reason for limiting the conditions for manufacturing the steel sheet according to the present embodiment will be described.

The present inventors are confirming that the steel sheet according to the present embodiment can be obtained by a manufacturing method including a heating step, a hot rolling step, a cooling step and a coiling step as described below.

Heating Step

First, a slab or steel piece having the above-mentioned chemical composition is heated. The slab or steel piece may be a slab or steel piece obtained by continuous casting or casting and blooming or may be also a slab or steel piece obtained by additionally performing hot working or cold working on the above-described slab or steel piece.

Retention Time in Temperature Range of 700° C. to 850° C. During Heating: 900 Seconds or Longer

When the slab or steel piece that is to be subjected to hot rolling is heated, the slab or steel piece is caused to retain in a temperature range of 700° C. to 850° C. for 900 seconds or longer. In austenitic transformation occurring in the temperature range of 700° C. to 850° C., Mn is distributed to ferrite and austenite. Therefore, when the transformation time is extended by extending the retention time, it is possible to diffuse Mn in the ferrite region. This eliminates Mn microsegregation that is unevenly distributed in the slab and significantly reduces the standard deviation of the Mn concentration.

Heating Temperature: 1280° C. or Higher and SRT (°C) or Higher

The heating temperature of the slab or steel piece that is to be subjected to hot rolling is set to 1280° C. or higher and a temperature SRT (°C) represented by the following formula (3) or higher. When the heating temperature is lower than 1280° C., there is a case where the reduction in the standard deviation of the Mn concentration due to the diffusion of Mn during heating becomes insufficient. In addition, the heating temperature is lower than the SRT (°C), the solutionizing of a Ti carbonitride becomes insufficient, and, in any cases, the tensile strength or bending workability of the steel sheet deteriorates. Therefore, the temperature of the slab or steel piece that is to be subjected to hot rolling is set to 1280° C. or higher and the SRT (°C) or higher. Here, the fact that “the temperature of the slab or steel piece is 1280° C. or higher and the SRT (°C) or higher” means that the temperature of the slab or steel piece is higher than the higher temperature of 1280° C. and the SRT (°C) or the higher temperature of 1280° C. and the SRT (°C) is the same as the temperature of the slab or steel piece.

On the other hand, when the heating temperature is higher than 1400° C., there is a case where a thick scale is formed to decrease the yield or significantly damage heating furnaces. Therefore, the heating temperature is preferably 1400° C. or lower.

Here, [element symbol] in the formula (3) indicates the amount of each element by mass%.

Hot Rolling Step

In the hot rolling step, multi-pass hot rolling is performed on the slab or steel piece after the heating step using a plurality of rolling stands to produce a hot-rolled steel sheet. The hot rolling step is divided into rough rolling and finish rolling that is performed after the rough rolling.

The multi-pass hot rolling can be performed using a reverse mill or a tandem mill; however, at least several stages from the end are preferably performed using a tandem mill from the viewpoint of the industrial productivity.

Time From Beginning of Rough Rolling to Completion of Finish Rolling: 600 Seconds or Shorter

Since rolling promotes the precipitation of the Ti-based carbide and makes the precipitation begin, when the time taken until the completion of the finish rolling is too long, a large amount of a coarse Ti-based carbide is precipitated in austenite. In this case, a fine Ti-based carbide that contributes to high-strengthening and is precipitated in ferrite after the finish rolling reduces, the tensile strength of the steel sheet significantly decreases, and the bending workability deteriorates. Therefore, the time from the beginning of the rough rolling to the completion of the finish rolling is set to 600 seconds or shorter. The time is preferably 500 seconds or shorter, more preferably 400 seconds or shorter and most preferably 320 seconds or shorter.

Normally, in hot rolling steps, the rolling reduction and the rolling temperature are controlled depending on the specification of a roller, the sheet thickness and sheet width of a coil to be manufactured and a desired material, but the time from the beginning of rough rolling to the completion of finish rolling is not comprehensively controlled. The present inventors newly found that the time from the beginning of the rough rolling to the completion of the finish rolling affects the precipitation state of the Ti-based carbide.

Total Rolling Reduction Within Temperature Range of 850° C. to 1100° C.: 90% or Larger

When hot rolling is performed in a manner that the total rolling reduction within a temperature range of 850° C. to 1100° C. becomes 90% or larger, mainly recrystallized austenite is refined, and the accumulation of the strain energy in the non-recrystallized austenite is promoted. As a result, the recrystallization of austenite is promoted, the diffusion of Mn atoms is promoted, and the standard deviation of the Mn concentration becomes small. Therefore, in the hot rolling, the total rolling reduction (cumulative rolling reduction) within the temperature range of 850° C. to 1100° C. is set to 90% or larger.

The total rolling reduction within the temperature range of 850° C. to 1100° C. can be represented by (t0 - t1)/t0 × 100 (%) where the inlet sheet thickness before the first pass in rolling within this temperature range is indicated by t0 and the outlet sheet thickness after the final pass in the rolling within this temperature range is indicated by t1.

Finish Rolling Completion Temperature FT (°C): TR (°C) or Higher and 1080° C. or Lower

When the FT (°C) is lower than TR (°C) represented by the following formula (4), significantly flat austenite is formed before cooling after the finish rolling, the microstructure is elongated in the rolling direction in the final product steel sheet, the average aspect ratio of crystal grains excluding residual austenite and having a bcc structure becomes smaller, and the plastic anisotropy becomes large. In this case, the elongation, stretch flangeability and/or bending workability of the steel sheet deteriorates. Therefore, the FT (°C) is set to the TR (°C) or higher.

On the other hand, when the FT (°C) exceeds 1080° C., the structure becomes coarse, and the bending workability of the steel sheet deteriorates. Therefore, the FT (°C) is set to 1080° C. or lower. The FT (°C) is preferably 1060° C. or lower.

The temperature during the finish rolling refers to the surface temperature of steel and can be measured with a radiation-type thermometer or the like.

Here, [element symbol] in the formula (4) indicates the amount of each element by mass%, and zero is assigned in a case where the corresponding element is not contained.

Cooling Step

The method for manufacturing the steel sheet according to the present embodiment has, as the next step of the hot rolling step, a cooling step of cooling the hot-rolled steel sheet with water to a temperature range of 650° C. to 800° C. at an average cooling rate of 45° C./second or faster. In addition, in the method for manufacturing the steel sheet according to the present embodiment, the cooling step is begun within 3.0 seconds after the end of the hot rolling step (after the completion of the finish rolling).

Time From Completion of Finish Rolling to Beginning of Water Cooling: 3.0 Seconds or Shorter

When the time from the completion of the finish rolling to the beginning of water cooling is longer than 3.0 seconds, the tensile strength or the bending workability deteriorates due to the growth of the refined austenite crystal grains or the coarse precipitation of a carbonitride of Ti or the like. Therefore, in the method for manufacturing the steel sheet according to the present embodiment, the water cooling is begun within 3.0 seconds after the completion of the finish rolling. The time is preferably 2.0 seconds or shorter and more preferably 1.5 seconds or shorter.

Average Cooling Rate from Beginning of Water Cooling After Completion of Finish Rolling to Water Cooling Stop Temperature of 650° C. to 800° C.: 45° C./Second or Faster

When the average cooling rate to a water cooling stop temperature of 650° C. to 800° C. is slower than 45° C./second, a coarse Ti-based carbide is precipitated in non-transformed austenite or in transformed ferrite grains, and it becomes difficult to obtain a desired strength. Therefore, the average cooling rate is set to 45° C./second or faster. The average cooling rate is preferably 50° C./second or faster and more preferably 55° C./second or faster. The upper limit is not particularly limited, but is preferably 300° C./second or slower from the viewpoint of the facility cost. The average cooling rate is a value obtained by dividing the amount of temperature dropped from the beginning of the water cooling after the completion of the hot rolling to the stopping of the water cooling by the required time.

Retention Time Within Temperature Range of 650° C. to 800° C.: 5 to 50 Seconds

After cooled to 650° C. to 800° C. at an average cooling rate of 45° C./second or faster, the steel sheet is caused to retain in the corresponding temperature range. When the retention time at 650° C. to 800° C. is short, since it becomes difficult to obtain a desired ferrite area fraction, the retention time needs to be five seconds or longer. The retention time is preferably seven seconds or longer. On the other hand, when the retention time is long, pearlite is formed, and the hole expandability deteriorates. Therefore, the retention time within this temperature range is set to 50 seconds or shorter. The retention time is preferably 40 seconds or shorter.

In addition, while the steel sheet is caused to retain at 650° C. to 800° C., ferritic transformation progresses, and a Ti-based carbide having a semi-coherent interface is precipitated in ferrite. As the results, a steel sheet being excellent in terms of the tensile strength and the hole expandability can be obtained. When the Ti-based carbide is precipitated at a temperature higher than 800° C., the Ti-based carbide is coarsely precipitated, a desired number density cannot be obtained, and it becomes difficult to obtain a desired tensile strength. On the other hand, when the Ti-based carbide is precipitated at a temperature lower than 650° C., a Ti-based carbide having a coherent interface is precipitated, and the hole expandability deteriorates.

Average Cooling Rate Within Temperature Range of 550° C. to 650° C.: 45° C./Second or Faster

After the retention, the steel sheet is cooled to a temperature of 550° C. or lower (coiling temperature in a manner that the average cooling rate within a temperature range of 550° C. to 650° C. becomes 45° C./second or faster. When the average cooling rate is slower than 45° C./second, a Ti-based carbide having a coherent interface is precipitated during the cooling, and the hole expandability deteriorates. The upper limit of the average cooling rate is not particularly limited, but is preferably 300° C./second or slower from the viewpoint of the facility cost.

Coiling Step Coiling Temperature: 350° C. or Higher and Lower Than 550° C.

After the cooling step, the steel sheet is coiled at 350° C. or higher and lower than 550° C. When the coiling temperature is lower than 350° C., non-transformed austenite transforms into martensite, and the hole expandability or the bending workability deteriorates. On the other hand, when the coiling temperature becomes 550° C. or higher, a Ti-based carbide having a coherent interface is formed after the coiling, and the hole expandability deteriorates. The coiling temperature is preferably 400° C. or higher and lower than 500° C.

In the present embodiment, a plated steel sheet having a plating layer may be produced by performing plating on the surface of the steel sheet after the coiling step. Even in a case where plating is performed, there is no problem in performing the plating as long as the conditions for the method for manufacturing the steel sheet according to the present embodiment are satisfied. The plating may be any of electroplating and hot-dip plating, and the plating type is also not particularly limited, but is ordinarily zinc-based plating including zinc plating and zinc alloy plating. As examples of the plated steel sheet, an electrolytic zinc-plated steel sheet, an electrolytic zinc-nickel alloy-coated steel sheet, a hot-dip galvanized steel sheet, a galvannealed steel sheet, a hot-dip zinc-aluminum alloy-coated steel sheet and the like are exemplary examples. The plating adhesion amount may be an ordinary amount. Before the plating, Ni or the like may be applied to the surface as pre-plating.

At the time of manufacturing the steel sheet according to the present embodiment, well-known temper rolling may be performed as appropriate for the purpose of shape correction.

The sheet thickness of the steel sheet according to the present embodiment is not particularly limited, but is preferably 8.0 mm or less since, in a case where the sheet thickness is too thick, microstructures formed in the surface layer and the inside of the steel sheet significantly differ. The sheet thickness is more preferably 6.0 mm or less. On the other hand, when the sheet thickness is too thin, since threading during hot rolling becomes difficult, ordinarily, the sheet thickness is preferably 1.0 mm or more. The sheet thickness is more preferably 1.2 mm or more.

EXAMPLES

Next, the effect of one aspect of the present invention will be more specifically described using examples, but conditions in the examples are simply examples of the conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to these examples of the conditions. The present invention is capable of adopting a variety of conditions within the scope of the gist of the present invention as long as the object of the present invention is achieved.

Steel materials having a chemical composition (unit mass%, the remainder was Fe and impurities) shown in Table 1A and Table 1B and having a sheet thickness of 250 mm were hot-rolled under conditions shown in Table 2A and Table 2B, thereby producing hot-rolled steel sheets having a sheet thickness of 2.5 to 3.5 mm. On some of the obtained hot-rolled steel sheets, a hot-dip galvanizing treatment with an annealing temperature of 700° C. and, furthermore, an alloying treatment were performed to produce hot-dip galvanized steel sheets (GI) or galvannealed steel sheets (GA).

TABLE 1A Steel (Mass%: remainder is Fe and impurities) C Si Mn P S sol. Al Ti N Nb V Mo A 0.082 0.620 1.32 0.015 0.0020 0.06 0.310 0.0024 B 0.062 0.052 1.35 0.010 0.0012 0.05 0.225 0.0033 C 0.120 1.561 1.32 0.013 0.0019 0.08 0.185 0.0045 D 0.084 0.053 1.29 0.011 0.0009 0.09 0.316 0.0039 E 0.081 0.064 1.32 0.009 0.0026 0.05 0.297 0.0032 0.025 F 0.088 0.111 1.42 0.010 0.0017 0.07 0.248 0.0043 0.106 G 0.087 0.042 1.40 0.009 0.0012 0.05 0.278 0.0032 0.110 H 0.084 0.063 1.31 0.014 0.0002 0.06 0.311 0.0034 I 0.082 0.025 1.30 0.010 0.0027 0.09 0.313 0.0037 J 0.072 0.079 1.25 0.016 0.0020 0.03 0.302 0.0044 K 0.076 0.368 1.35 0.010 0.0027 0.04 0.279 0.0037 L 0.075 0.032 1.36 0.016 0.0018 0.06 0.279 0.0029 M 0.075 0.025 1.40 0.010 0.0024 0.04 0.285 0.0030 N 0.073 0.039 1.32 0.010 0.0019 0.08 0.291 0.0038 O 0.075 0.025 2.56 0.012 0.0015 0.05 0.262 0.0031 0.021 P 0.071 0.102 0.68 0.008 0.0021 0.03 0.283 0.0034 Q 0.041 0.209 1.38 0.014 0.0017 0.07 0.256 0.0030 R 0.052 0.498 1.26 0.016 0.0006 0.08 0.138 0.0041 S 0.125 0.046 1.35 0.011 0.0026 0.07 0.450 0.0031 I 0.208 0.517 1.30 0.015 0.0027 0.05 0.252 0.0039 U 0.083 0.498 3.25 0.014 0.0030 0.06 0.301 0.0034 V 0.091 0.045 1.32 0.011 0.0015 0.09 0.292 0.0025 0.028 W 0.125 0.510 0.52 0.012 0.0021 0.05 0.225 0.0025 0.020 0.235 0.104 X 0.082 0.025 1.30 0.010 0.0027 0.09 0.320 0.0037 Y 0.078 0.201 1.58 0.015 0.0032 0.36 0.251 0.0021 Z 0.088 0.950 1.72 0.008 0.0036 0.07 0.275 0.0036

TABLE 1B Steel (Mass%: remainder is Fe and impurity) ex. C (T) SRT (°C) TR (°C) Cu Ni Cr W B Ca Mg REM Bi A 0.007 1300 924 B 0.009 1245 892 C 0.078 1287 876 D 0.009 1303 927 E 0.007 1295 934 F 0.005 1286 900 G 0.007 1295 912 H 0.06 0.04 0.009 1302 925 I 0.20 0.008 1300 926 J 0.0022 0.001 1285 921 K 0.0015 0.010 1283 912 L 0.0023 0.008 1282 912 M 0.0021 0.007 1284 915 N 0.0018 0.004 1283 917 O 0.010 1276 918 P 0.004 1278 914 Q -0.020 1220 904 R 0.021 1186 858 S 0.016 1371 978 T 0.149 1365 902 U 0.012 1298 921 V 0.017 1303 934 W 0.001 1309 903 X 0.050 0.003 1302 928 Y 0.10 0.06 0.0020 0.0015 0.018 1276 902 Z 0.101 0.0025 0.017 1295 911

TABLE 2A Test No. Steel Retention time at 700° C. to 850° C. during heating (seconds) Heating temperature (°C) Time from beginning of rough rolling to completion of finish rolling (seconds) Total rolling reduction in temperature range of 850° C. to 1100° Finish rolling completion temperature FT (°C) Time from finish rolling to beginning of water cooling (seconds) 1 A 1137 1383 431 91 951 0.8 2 A 1242 1375 330 95 957 2.4 3 A 1132 1373 359 93 952 1.6 4 A 1227 1380 320 95 954 1.0 5 A 1145 1375 557 94 965 2.3 6 A 834 1385 431 95 943 1.1 7 A 1485 1228 446 95 927 0.9 8 A 1518 1380 682 95 949 1.6 9 A 1562 1373 469 83 932 1.1 10 A 1469 1380 314 94 847 0.8 11 A 1084 1380 328 93 958 4.1 12 A 1202 1376 321 93 942 0.9 13 A 1010 1365 305 93 940 1.2 14 A 989 1383 337 94 941 1.2 15 A 993 1365 342 94 951 1.3 16 B 1120 1348 418 95 902 1.3 17 C 1463 1350 322 94 905 1.1 18 D 1272 1380 353 95 977 1.3 19 E 1532 1350 308 94 982 1.4 20 F 1070 1350 418 92 918 0.9 21 G 1285 1350 402 93 922 1.3 22 H 1142 1350 390 93 953 1.4 23 I 1052 1350 261 94 939 1.2 24 J 1317 1350 355 96 953 1.2 25 K 1194 1350 317 94 931 1.3 26 L 1500 1350 263 95 936 1.4 27 M 1336 1350 344 93 933 1.3 28 N 1563 1350 378 95 944 1.6 29 O 1133 1350 281 93 937 1.5 30 P 1484 1350 408 95 946 1.2 31 Q 1310 1350 362 94 920 1.4 32 R 1148 1350 391 93 887 1.1 33 S 1102 1380 375 93 989 1.6 34 T 978 1380 276 93 923 1.3 35 U 1210 1350 301 93 956 1.8 36 V 1023 1350 305 93 964 1.5 37 W 1065 1380 318 93 952 1.8 37 X 1253 1380 400 93 942 1.2 38 Y 1546 1350 293 94 963 0.7 39 Z 1032 1350 284 92 951 0.5

TABLE 2B Test No. Cooling rate from beginning of water cooling after completion of finish rolling to water cooling stop temperature of 650° C. to 850° C. (°C/s) Retention time at 650° C. to 800° C. (seconds) Average cooling rate from 550° C. to 650° C. (°C/s) Coiling temperature (°C) SRT (°C) TR (°C) 1 87 7 88 402 1300 924 2 80 18 89 450 1300 924 3 54 12 68 443 1300 924 4 75 11 69 456 1300 924 5 71 12 62 489 1300 924 6 77 12 57 415 1300 924 7 60 12 62 451 1300 924 8 56 12 56 431 1300 924 9 60 16 66 426 1300 924 10 58 16 64 476 1300 924 11 62 12 51 431 1300 924 12 12 15 80 425 1300 924 13 68 3 71 425 1300 924 14 59 14 0.1 625 1300 924 15 65 14 1 456 1300 924 16 50 12 69 482 1245 892 17 72 16 94 465 1287 876 18 57 11 66 395 1303 927 19 77 15 76 415 1295 934 20 80 13 80 426 1286 900 21 87 14 87 457 1295 912 22 88 12 77 432 1302 925 23 71 11 71 419 1300 926 24 76 18 66 427 1285 921 25 79 14 85 461 1283 912 26 81 12 68 428 1282 912 27 68 16 69 409 1284 915 28 83 15 102 428 1283 917 29 52 35 59 437 1276 918 30 59 11 79 451 1278 914 31 82 11 97 468 1220 904 32 60 13 81 456 1186 858 33 72 13 62 451 1371 978 34 74 40 66 465 1365 902 35 55 45 59 468 1298 921 36 68 14 84 410 1303 934 37 80 13 89 459 1309 903 37 85 15 92 432 1302 928 38 65 14 54 410 1276 902 39 70 16 58 489 1295 911

Regarding the obtained steel sheets (the hot-rolled steel sheets and the plated steel sheets), the microstructures at the ¼ depth positions of the sheet thicknesses from the surfaces of the steel sheets were observed, and the area fractions of individual structures, the average crystal grain diameters and average aspect ratios of the crystal grains having a bcc structure and the standard deviations of the Mn concentrations were obtained.

The area fractions of the microstructure at the ¼ depth position of the sheet thickness from the surface of the steel sheet, the average crystal grain diameter and average aspect ratio of the crystal grains having a bcc structure were obtained by the scanning electron microscopic (SEM) observation and electron back scattering diffraction (EBSD) analysis of the microstructure at the ¼ depth position of the sheet thickness from the surface of the steel sheet of a cross section of the steel sheet parallel to a rolling direction and the sheet thickness direction using an EBSD analyzer composed of a thermal field emission scanning electron microscope and an EBSD detector.

At that time, in a region that was 200 µm long in the rolling direction and 100 µm long in the sheet thickness direction and had the ¼ depth position of the sheet thickness from the surface of the steel sheet at the center, crystal orientation information was acquired at 0.2 µm intervals while differentiating fcc and bcc. Crystal grain boundaries having a crystal orientation difference of 15° or more were specified using the software attached to the EBSD analyzer (“OIM Analysis (registered trademark)” manufactured by AMETEK, Inc.). Regarding the average crystal grain diameter of bcc, the average crystal grain diameter was obtained by defining a region that was surrounded by crystal grain boundaries having a crystal orientation difference of 15° or more, was identified as bcc and had a circle equivalent diameter of 0.3 µm or more as a crystal grain.

The area fraction of ferrite was measured by the following method.

A region that was surrounded by crystal grain boundaries having a crystal orientation difference of 5° or more, was identified as bcc and had a circle equivalent diameter of 0.3 µm or more was defined as a crystal grain. Among such crystal grains, for crystal grains for which a value that was obtained by an analysis with Grain Average Misorientation analysis equipped in OIM Analysis (GAM value) was 0.6° or less, the area fraction was calculated.

The area fraction of pearlite and cementite was obtained by the point counting method with lattice spacings of 5 µm after the microstructure revealed by Nital etching in a region at the ¼ depth position of the sheet thickness from the surface of the steel sheet was observed at three visual fields using a SEM at a magnification of 1000 times. In addition, the area fraction of MA was obtained by the point counting method with lattice spacings of 5 µm after the structure revealed by LePera etching in the region at the ¼ depth position of the sheet thickness from the surface of the steel sheet was observed at two visual fields using an optical microscope at a magnification of 500 times.

While not shown in the table, the remainders of the microstructures were bainite.

The standard deviation of the Mn concentration was obtained by mirror-polishing a cross section of the steel sheet parallel to the rolling direction and the sheet thickness direction and then measuring the ¼ depth position of the sheet thickness from the surface of the steel sheet with an electron probe microanalyzer (EPMA). As the measurement conditions, the acceleration voltage was set to 15 kV, the magnification was set to 5000 times, and a distribution image in a range that was 20 µm long in the sample rolling direction and 20 µm long in the sample sheet thickness direction was measured. More specifically, the measurement intervals were set to 0.1 µm, and the Mn concentrations at 40000 or more sites were measured. Next, the standard deviation was calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentration.

In order to evaluate the mechanical properties of the obtained steel sheets, the tensile strengths TS (MPa) and the total elongations at fracture El (%) were measured according to JIS Z 2241: 2011. In addition, the limiting hole expansion ratios (λ) were measured according to JIS Z 2256: 2010.

The bending workability was evaluated by a 90° V bend test in which the bend radius was set to twice the sheet thickness.

Table 3A and Table 3B show the microstructures and the test results of the mechanical properties.

The tensile strength was evaluated as a high strength in a case where the tensile strength was 980 MPa or more.

The elongation was evaluated as excellent in a case where the product of the tensile strength and the total elongation at fracture (TS x El) was 14000 MPa·% or more. In addition, in a case where TS × λ was 50000 MPa·% or more, the stretch flangeability was evaluated as excellent. The bending workability was evaluated as excellent bending workability (OK) when cracking did not occur in all test pieces during the bend test performed three times and evaluated as insufficient bending workability (NG) when cracking occurred in one or more test pieces.

TABLE 3A Test No. Steel Microstructure Ferrite area fraction (%) MA area fraction (%) Pearlite and cementite area fraction (%) Average crystal grain diameter (µm) Average aspect ratio Standard deviation of Mn concentration (mass%) Coherent/semi-coherent 1 A 62 2 0 6.2 0.61 0.52 Semi-coherent 2 A 95 0 0 5.8 0.60 0.42 Semi-coherent 3 A 77 0 0 6.5 0.59 0.45 Semi-coherent 4 A 74 0 0 6.1 0.61 0.43 Semi-coherent 5 A 71 0 0 6.7 0.58 0.43 Semi-coherent 6 A 77 1 0 5.2 0.56 0.71 Semi-coherent 7 A 83 0 0 6.2 0.41 0.65 Semi-coherent 8 A 74 0 0 6.2 0.58 0.27 Semi-coherent 9 A 76 0 0 6.0 0.51 0.64 Semi-coherent 10 A 90 0 0 7.3 0.12 0.31 Semi-coherent 11 A 78 0 0 10.5 0.66 0.45 Semi-coherent 12 A 89 0 0 10.3 0.56 0.45 Semi-coherent 13 A 5 5 0 8.5 0.54 0.46 Semi-coherent 14 A 99 0 1 8.1 0.58 0.40 Coherent 15 A 84 0 0 8.6 0.58 0.42 Coherent 16 B 84 0 0 6.1 0.56 0.34 Semi-coherent 17 C 69 1 6 5.4 0.56 0.28 Semi-coherent 18 D 78 0 0 6.1 0.68 0.29 Semi-coherent 19 E 89 0 0 5.6 0.65 0.22 Semi-coherent 20 F 80 0 0 5.3 0.62 0.45 Semi-coherent 21 G 81 0 0 6.3 0.58 0.42 Semi-coherent 22 H 79 0 0 5.8 0.60 0.41 Semi-coherent 23 I 78 0 0 6.8 0.56 0.38 Semi-coherent 24 J 100 0 0 6.3 0.57 0.33 Semi-coherent 25 K 85 0 0 6.0 0.59 0.42 Semi-coherent 26 L 81 0 0 7.0 0.62 0.26 Semi-coherent 27 M 91 0 0 5.8 0.62 0.30 Semi-coherent 28 N 90 0 0 5.6 0.59 0.31 Semi-coherent 29 O 78 0 0 6.1 0.61 0.39 Semi-coherent 30 P 98 0 0 5.6 0.56 0.29 Semi-coherent 31 Q 79 0 0 4.5 0.63 0.48 Semi-coherent 32 R 90 0 0 4.2 0.62 0.42 Semi-coherent 33 S 79 0 0 7.4 0.60 0.35 Semi-coherent 34 T 66 6 9 5.4 0.63 0.46 Semi-coherent 35 U 32 4 0 5.9 0.60 0.55 Semi-coherent 36 V 85 4 0 5.8 0.54 0.47 Semi-coherent 37 W 80 1 0 6.4 0.56 0.50 Semi-coherent 37 X 90 0 0 5.4 0.52 0.35 Semi-coherent 38 Y 78 0 1 6.2 0.63 0.31 Semi-coherent 39 Z 91 0 1 6.8 0.55 0.44 Semi-coherent

TABLE 3B Test No. Characteristics Plating Note TS (MPa) E1 (%) λ (%) TS × El (MPa·%) TS x λ (MPa·%) Bending workability 1 1210 11.9 46 14399 55660 OK - Invention Example 2 1120 15.7 78 17584 87360 OK - Invention Example 3 1194 13.2 55 15761 65670 OK GI Invention Example 4 1182 13.5 53 15957 62646 OK GA Invention Example 5 1052 14.2 55 14938 57860 OK - Invention Example 6 1192 13.1 55 15615 65560 NG - Comparative Example 7 845 18.0 71 15210 59995 NG - Comparative Example 8 954 16.1 69 15359 65826 NG - Comparative Example 9 1180 13.2 67 15576 79060 NG - Comparative Example 10 1085 15.1 35 16384 37975 NG - Comparative Example 11 985 15.5 60 15268 59100 NG - Comparative Example 12 932 17.5 75 16310 69900 OK - Comparative Example 13 958 10.5 65 10059 62270 OK - Comparative Example 14 1185 14.2 28 16827 33180 OK - Comparative Example 15 1199 14.4 36 17266 43164 OK - Comparative Example 16 1008 16.3 73 16430 73584 OK - Invention Example 17 1025 14.2 35 14555 35875 OK - Comparative Example 18 1308 12.8 51 16742 66708 OK - Invention Example 19 1170 14.9 68 17464 79531 OK - Invention Example 20 1138 14.5 60 16503 68286 OK - Invention Example 21 1158 14.5 61 16791 70638 OK - Invention Example 22 1214 13.9 56 16879 68000 OK - Invention Example 23 1195 14.2 57 16963 68090 OK - Invention Example 24 1204 15.1 82 18178 98717 OK - Invention Example 25 1211 14.2 61 17200 73886 OK - Invention Example 26 1096 15.3 63 16767 69040 OK - Invention Example 27 1166 15.2 70 17717 81592 OK - Invention Example 28 1082 15.9 74 17192 80088 OK - Invention Example 29 1298 12.2 51 15839 66214 OK - Invention Example 30 1033 17.1 88 17671 90938 OK - Invention Example 31 865 18.2 75 15743 64875 OK - Comparative Example 32 825 19.2 92 15840 75900 OK - Comparative Example 33 942 15.0 54 14130 50868 NG - Comparative Example 34 1002 15.2 23 15230 23046 NG - Comparative Example 35 1160 11.5 45 13340 52200 OK - Comparative Example 36 1189 14.5 44 17241 52316 OK - Invention Example 37 1265 13.1 55 16572 69575 OK - Invention Example 37 1212 14.9 58 18059 70296 OK - Invention Example 38 1181 13.1 54 15471 63774 OK - Invention Example 39 1189 13.9 52 16527 61828 OK - Invention Example

As shown in Table 3A and Table 3B, in the invention examples where the requirements of the present invention were satisfied, all of TS, TS × El and the bending workability were excellent. On the other hand, in the comparative example where at least one of the requirements of the present invention was not satisfied, at least one of TS, TS × El and the bending workability was poor.

Industrial Applicability

According to the present invention, it is possible to provide a steel sheet having a high strength and being excellent in terms of elongation, stretch flangeability and bending workability. The steel sheet of the present invention is preferable as a material that is used in uses for automobiles, home appliances, mechanical structures, construction and the like, and, in particular, when the steel sheet is used as a material for components such as inner sheet members, structural members, suspension members, and the like of automobiles, not only is a contribution made to weight reduction in vehicle bodies and improvement in impact resistance but the steel sheet is also easily worked into component shapes. Therefore, the steel sheet of the present invention makes an extreme industrial contribution. 

1-5. (canceled)
 6. A steel sheet comprising, as a chemical composition, by mass%: C: 0.050% to 0.250%; Si: 0.005% to 2.000%; Mn: 0.10% to 3.00%; P: 0.100% or less; S: 0.0100% or less; sol. Al: 0.001% to 1.00%; Ti: 0.150% to 0.400%; N: 0.0010% to 0.0100%; Nb: 0% to 0.100%; V: 0% to 1.000%; Mo: 0% to 1.000%; Cu: 0% to 1.00%; Ni: 0% to 1.00%; Cr: 0% to 2.00%; W: 0% to 1.000%; B: 0% to 0.0020%; Ca: 0% to 0.0100%; Mg: 0% to 0.0100%; REM: 0% to 0.0100%; and Bi: 0% to 0.0200% with a remainder of Fe and impurities, wherein Ex. C obtained by the following formula (1) is 0.020% or less, a microstructure at a ¼ depth position of a sheet thickness from a surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder of bainite in terms of area fractions, in the microstructure, an average crystal grain diameter is 10.0 µm or less, an average aspect ratio of crystal grains is 0.30 or more, a standard deviation of a Mn concentration is 0.60 mass% or less, a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state, and a tensile strength is 980 MPa or more, Ex. C = (%C) - 12{(%Ti*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184} Formula (1) here, “%Ti*” in the formula (1) is obtained from the following formula (2), %Ti* = %Ti - 48 × {(%N)/14 + (%S)/32) Formula (2) %C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%.
 7. The steel sheet according to claim 6, comprising, as the chemical composition, by mass%, one or more selected from: Nb: 0.001% to 0.100%; V: 0.005% to 1.000%; Mo: 0.001% to 1.000%; Cu: 0.02% to 1.00%; Ni: 0.02% to 1.00%; Cr: 0.02% to 2.00%; W: 0.02% to 1.000%; B: 0.0001% to 0.0020%; Ca: 0.0002% to 0.0100%; Mg: 0.0002% to 0.0100%; REM: 0.0002% to 0.0100%; and Bi: 0.0001% to 0.0200%.
 8. The steel sheet according to claim 6, wherein a plating layer is formed on a surface.
 9. The steel sheet according to claim 7, wherein a plating layer is formed on a surface.
 10. The steel sheet according to claim 8, wherein the plating layer is a hot-dip galvanized layer.
 11. The steel sheet according to claim 9, wherein the plating layer is a hot-dip galvanized layer.
 12. The steel sheet according to claim 10, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.
 13. The steel sheet according to claim 11, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer. 